The concept of a wet well/dry well combination in a common structure is not novel. Dry wells are usually large areas that permit the entrance of a person to work on the components of a pumping system. Dry wells can be located at a fixed distance from the bottom of the sump where a pump is suspended through the floor of the dry well into the wet well below, hanging a given distance off of the bottom of the wet well. A deeper dry well is created as the depth of the basin increases due to such factors as the depth of the frostline. With today's `confined space` work safety requirements and concerns with toxic fumes, the need to prevent unnecessary confined space entry and exposure to potentially hazardous environments is a major area of public interest and potential legal liability. It is well known that persons have been overcome by toxic fumes working in wet well environments, some with fatal consequences.
A typical basin for this sewage application is made from fiberglass or polyethylene and is simply a wet well, with electrical junction boxes located inside the basin and a control panel located on the outside surface. Most pumping stations use mechanical floats as fluid level controls, which have a tendency to malfunction either because they become fouled with grease, wedged stuck or tangled with other equipment. The floats require regular maintenance.
One prior embodiment of a residential sewage pumping system uses a wet well/dry well basin design. In this embodiment, the pump is bolted through a center shelf located at a fixed distance from the bottom so that the pump is suspended off the bottom. In order to accommodate different requirements for burial depth, extensions are added, increasing the depth of the dry well. A person servicing this type of system may be required to work inside the dry well to unbolt the pump and lift it up. As soon as the pump is removed, the dry well technically becomes part of the wet well, as there is nothing separating the two chambers. This becomes more of a problem when a person must reposition the pump onto the center shelf and bolt it back into place. Another problem with this particular system is the inability to physically watch the pump and level controller functioning.
Another potential problem with buried sewage sumps is the typically rigid piping-to-sump connection, which can be exposed to some large shear stresses as the basin shifts in the ground due to back filling, basin settling and/or hydrostatic lifting forces.
Many pumping system basins use a guide rail system for positioning the pump and holding it in place. The guide rails are typically bolted through the wall of the basin, which can act as a weak point for failure of the basin wall or for the development of future leaks.